The long-term objective of this research program is to explain how children develop a theory of mind, that is, an understanding of the causal relationships between experiences, mental states, and behavior. A theory of mind is necessary for mature social cognition and healthy social functioning. The specific aims of this research program are to compare the development of knowledge about visual perception and vision as a source of knowledge in deaf children and hearing children, in order to determine whether differences in perceptual, linguistic, or cultural experience affect the course of development. Deaf children of deaf parents, hearing children of deaf parents, and hearing children of hearing parents, ages 3 to 5 years, will be tested on a range of tasks designed to measure their ability to assess what other people perceive (visual perspective-taking) and what other people believe as a result of what they perceive (conceptual perspective-taking).